Not common, but technically correct - the sentences about "being człowiek/ludzie" without any adjective (tall, short, rich etc.) are like talking about the species in a sci-fi context.
In the beginning of the course we have little vocabulary and little grammar to choose from, so we end up with such sentences.
And anyway, it's only an accepted option here, not the main one.
I am struggling to grasp the difference between instrumental and nominative, along with all the other cases. I get that nominative is 'to be', but explaining instrumental by using the word 'instrument' in it's description really doesn't help me grasp it. Could someone put it in simple terms for a simple Englishman please?
One grammatical case can have a dozen different applications, so it's impossible to find a name that suits all of them. Therefore you shouldn't focus on the name that much.
Here's a guide on when to use instrumental vs. nominative in sentences like these:
The reason why it's called 'instrumental' is because one main application is denoting the mean of an action:
- Jadę samochodem. - I'm going by car.
- Kroję mięso nożem - I'm slicing meat with a knife.
There are also some verbs (many of them are reflexive) that require its object to take the instrumental case:
- Interesuję się koszykówką. - I'm interested in Basketball.
- Kawą nie pogardzę. - I won't say no to cup of coffee.
Furthermore, many prepositions require the instrumental case, like:
Z (meaning with), pod (meaning under), przed (meaning before), za (meaning behind)...